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    Earthquake, shaking of the earth’s surface caused by

    rapid movement of the earth’s rocky outer layer. Earthquakes occur when energy stored within

    the earth, usually in the form of strain in rocks, suddenly releases. This energy is transmitted to

    the surface of the earth by earthquake waves. The study of earthquakes and the waves they create

    is called seismology. Scientists who study earthquakes are called seismologists. (Webster’s

    p.423) The destruction an earthquake causes, depends on its magnitude or the amount of shaking

    that occurs. The size varies from small imperceptible shaking, to large shocks felt miles around.

    Earthquakes can tear up the ground, make buildings and other structures collapse, and create

    tsunamis (large sea waves). Many Lives can be lost because of this destruction. (The Road to

    Jaramillo p.211) Several hundred earthquakes, or seismic tremors, occur per day around the

    world. A worldwide network of seismographs detect about one million small earthquakes per

    year. Very large earthquakes, such as the 1964 Alaskan earthquake, which measured 8.6 on the

    Richter scale and caused millions of dollars in damage, occur worldwide once every few years.

    Moderate earthquakes, such as the 1989 tremor in Loma Prieta, California (magnitude 7.0), and

    the 1995 tremor in Kôbe, Japan (magnitude 6.8), occur about 20 times a year. Moderate

    earthquakes also cause millions of dollars in damage and can harm many people. (The Road to

    Jaramillo p.213-215) In the last 500 years, several million people have been killed by

    earthquakes around the world, including over 240,000 in the 1976 T’ang-Shan, China,

    earthquake. Worldwide, earthquakes have also caused severe property and structural damage.

    Good precautions, such as education, emergency planning, and constructing stronger, more

    flexible structures, can limit the loss of life and decrease the damage caused by earthquakes.

    (The Road to Jaramillo p.213-215,263) AN EARTHQUAKES ANATOMY Seismologists

    examine the parts of an earthquake, like what happens to the earth’s surface during an

    earthquake, how the energy of an earthquake moves from inside the earth to the surface, and

    how this energy causes damage. By studying the different parts and actions of earthquakes,

    seismologists learn more about their effects and how to predict ground shaking in order to

    reduce damage. (On Shifting Ground p.109-110) Focus and Epicenter The point within the earth

    along the rupturing geological fault where an earthquake originates is called the focus, or

    hypocenter. The point on the earth’s surface directly above the focus is called the epicenter.

    Earthquake waves begin to radiate out from the focus and follow along the fault rupture. If the

    focus is near the surface between 0 and 70 km (0 and 40 mi.) deep shallow focus earthquakes are

    produced. If it is deep below the crust between 70 and 700 km (40 and 400 mi.) deep a deep

    focus earthquake will occur. Shallow-focus earthquakes tend to be larger, and therefore more

    damaging, earthquakes. This is because they are closer to the surface where the rocks are

    stronger and build up more strain. (The Ocean of Truth p.76 & The road to Jaramillo p.94-97)

    Seismologists know from observations that most earthquakes originate as shallow-focus

    earthquakes and most of them occur near plate boundaries areas where the earth’s crustal plates

    move against each other. Other earthquakes, including deep-focus earthquakes, can originate in

    subduction zones, where one tectonic plate subducts, or moves under another plate. (The Ocean

    of Truth p.54-56) I Faults Stress in the earth’s crust creates faults places where rocks have

    moved and can slip, resulting in earthquakes. The properties of an earthquake depend strongly

    on the type of fault slip, or movement along the fault, that causes the earthquake. Geologists

    categorize faults according to the direction of the fault slip. The surface between the two sides of

    a fault lies in a plane, and the direction of the plane is usually not vertical; rather it dips at an

    angle into the earth. When the rock hanging over the dipping fault plane slips downward into the

    ground, the fault is called a normal fault. When the hanging wall slips upward in relation to the

    bottom wall, the fault is called a reverse fault or a thrust fault. Both normal and reverse faults

    produce vertical displacements, or the upward movement of one side of the fault above the other

    side, that appear at the surface as fault scarps. Strike slip faults are another type of fault that

    produce horizontal displacements, or the side by side sliding movement of the fault, such as seen

    along the San Andreas fault in California. Strike-slip faults are usually found along boundaries

    between two plates that are sliding past each other. (Plate Tectonics p.49-53) II Waves The

    sudden movement of rocks along a fault causes vibrations that transmit energy through the earth

    in the form of waves. Waves that travel in the rocks below the surface of the earth are called

    body waves, and there are two types of body waves: primary, or P, waves, and secondary, or S,

    waves. The S waves, also known as shearing waves, cause the most damage during earthquake

    shaking, as they move the ground back and forth. (Plate tectonics p.133) Earthquakes also

    contain surface waves that travel out from the epicenter along the surface of the earth. Two types

    of these surface waves occur: Rayleigh waves, named after British physicist Lord Rayleigh, and

    Love waves, named after British geophysicist A. E. H. Love. Surface waves also cause damage

    to structures, as they shake the ground underneath the foundations of buildings and other

    structures. Body waves, or P and S waves, radiate out from the rupturing fault starting at the

    focus of the earthquake. P waves are compression waves because the rocky material in their path

    moves back and forth in the same direction as the wave travels alternately compressing and

    expanding the rock. P waves are the fastest seismic waves; they travel in strong rock at about 6

    to 7 km (4 mi.) per second. P waves are followed by S waves, which shear, or twist, rather than

    compress the rock they travel through. S waves travel at about 3.5 km (2 mi.) per second. S

    waves cause rocky material to move either side to side or up and down perpendicular to the

    direction the waves are traveling, thus shearing the rocks. Both P and S waves help seismologists

    to locate the focus and epicenter of an earthquake. As P and S waves move through the interior

    of the earth, they are reflected and refracted, or bent, just as light waves are reflected and bent by

    glass. Seismologists examine this bending to determine where the earthquake originated.

    (Encarta 98) On the surface of the earth, Rayleigh waves cause rock particles to move forward,

    up, backward, and down in a path that contains the direction of the wave travel. This circular

    movement is somewhat like a piece of seaweed caught in an ocean wave, rolling in a circular

    path onto a beach. The second type of surface wave, the Love wave, causes rock to move

    horizontally, or side to side at right angles to the direction of the traveling wave, with no vertical

    displacements. Rayleigh and Love waves always travel slower than P waves and usually travel

    slower than S waves. (The Floor of the Sea p.76-78, 112-115) III CAUSES Most earthquakes are

    caused by the sudden slip along geologic faults. The faults slip because of movement of the

    earth’s tectonic plates. This concept is called the elastic rebound theory. The rocky tectonic

    plates move very slowly, floating on top of a weaker rocky layer. As the plates collide with each

    other or slide past each other, pressure builds up within the rocky crust. Earthquakes occur when

    pressure within the crust increases slowly over hundreds of years and finally exceeds the strength

    of the rocks. Earthquakes also occur when human activities, such as the filling of reservoirs,

    increase stress in the earth’s crust. (Encarta 98) ELASTIC REBOUND THEORY In 1911

    American seismologist Harry Fielding Reid studied the effects of the April 1906 California

    earthquake. He proposed the elastic rebound theory to explain the generation of earthquakes that

    occur in tectonic areas, usually near plate boundaries. This theory states that during an

    earthquake, the rocks under strain suddenly break, creating a fracture along a fault. When a fault

    slips, movement in the crustal rock causes vibrations. The slip changes the local strain out into

    the surrounding rock. The change in strain leads to aftershocks, which are produced by further

    slips of the main fault or adjacent faults in the strained region. The slip begins at the focus and

    travels along the plane of the fault, radiating waves out along the rupture surface. On each side

    of the fault, the rock shifts in opposite directions. The fault rupture travels in irregular steps

    along the fault; these sudden stops and starts of the moving rupture give rise to the vibrations

    that propagate as seismic waves. After the earthquake, strain begins to build again until it is

    greater than the forces holding the rocks together, then the fault snaps again and causes another

    earthquake. (Plate tectonics p.56-59) DISTRIBUTION Seismologists have been monitoring the

    frequency and locations of earthquakes for most of the 20th century. They have found that the

    majority of earthquakes occur along plate tectonic boundaries, while there are relatively few

    intraplate earthquakes, that occur within a tectonic plate. The categorization of earthquakes is

    related to where they occur, as seismologists generally classify naturally occurring earthquakes

    into one of two categories: interplate and intraplate. Interplate earthquakes are the most

    common; they occur primarily along plate boundaries. Intraplate earthquakes occur within the

    plates at places where the crust is fracturing internally. Both interplate and intraplate earthquakes

    may be caused by tectonic or volcanic forces. (Naked Earth p.134-135) I Tectonic Earthquakes

    Tectonic earthquakes are caused by the sudden release of energy stored within the rocks along a

    fault. The released energy is produced by the strain on the rocks due to movement within the

    earth, called tectonic deformation. The effect is like the sudden breaking and snapping back of a

    stretched elastic band. (The Ocean of truth p.122) II Volcanic Earthquakes Volcanic earthquakes

    occur near active volcanoes but have the same fault slip mechanism as tectonic earthquakes.

    Volcanic earthquakes are caused by the upward movement of magma under the volcano, which

    strains the rock locally, and leads to an earthquake. As the fluid magma rises to the surface of the

    volcano, it moves and fractures rock masses and causes continuous tremors that can last up to

    several hours or days. Volcanic earthquakes occur in areas that are associated with volcanic

    eruptions, such as in the Cascade Mountain Range of the Pacific Northwest, Japan, Iceland, and

    at isolated hot spots such as Hawaii. (Plate tectonics p.74) LOCATIONS Seismologists use

    global networks of seismographic stations to accurately map the focuses of earthquakes around

    the world. After studying the worldwide distribution of earthquakes, the pattern of earthquake

    types, and the movement of the earth’s rocky crust, scientists proposed that plate tectonics, or the

    shifting of the plates as they move over another weaker rocky layer, was the main underlying

    cause of earthquakes. The theory of plate tectonics arose from several previous geologic theories

    and discoveries. Scientists now use the plate tectonics theory to describe the movement of the

    earth’s plates and how this movement causes earthquakes. They also use the knowledge of plate

    tectonics to explain the locations of earthquakes, mountain formation, deep ocean trenches, and

    predict which areas will be damaged the most by earthquakes. It is clear that major earthquakes

    occur most frequently in areas with features that are found at plate boundaries: high mountain

    ranges and deep ocean trenches. Earthquakes within plates, or intraplate tremors, are rare

    compared with the thousands of earthquakes that occur at plate boundaries each year, but they

    can be very large and damaging. (On shifting ground p.17-19) Earthquakes that occur in the area

    surrounding the Pacific Ocean, at the edges of the Pacific plate, are responsible for an average of

    80 percent of the energy released in earthquakes worldwide. Japan is shaken by more than 1000

    tremors greater than 3.5 in magnitude each year. The western coasts of North and South America

    are very also active earthquake zones, with several thousand small to moderate earthquakes each

    year. (U.S.G.S.) Intraplate earthquakes are less frequent than plate boundary earthquakes, but

    they are still caused by the internal fracturing of rock masses. The New Madrid, Missouri,

    earthquakes of 1811 and 1812 were extreme examples of intraplate seismic events. Scientists

    estimate that the three main earthquakes of this series were about magnitude 8.0 and that there

    were at least 1500 aftershocks. (The ocean of truth p.67-69) EFFECTS Ground shaking leads to

    landslides and other soil movement. These are the main damage causing events that occur during

    an earthquake. Primary effects that can accompany an earthquake include property damage, loss

    of lives, fire, and tsunami waves. Secondary effects, such as economic loss, disease, and lack of

    food and clean water, also occur after a large earthquake. (On shifting ground p.47) Ground

    Shaking and Landslides Earthquake waves make the ground move, shaking buildings and

    structures and causing poorly designed or weak structures partially or totally collapse. The

    ground shaking weakens soils and foundation materials under structures and causes dramatic

    changes in fine-grained soils. During an earthquake, water-saturated sandy soil becomes like

    liquid mud, an effect called liquefaction. Liquefaction causes damage as the foundation soil

    beneath structures and buildings weakens. Shaking may also dislodge large earth and rock

    masses, producing dangerous landslides, mudslides, and rock avalanches that may lead to loss of

    lives or further property damage. (The road to Jaramillo p.43-45) REDUCING DAMAGE

    Earthquakes cannot be prevented, but the damage they cause can be greatly reduced with

    communication strategies, proper structural design, emergency preparedness planning,

    education, and safer building standards. In response to the tragic loss of life and great cost of

    rebuilding after past earthquakes, many countries have established earthquake safety and

    regulatory agencies. These agencies require codes for engineers to use in order to regulate

    development and construction. Buildings built according to these codes survive earthquakes

    better and ensure that earthquake risk is reduced. (On shifting ground p.56) Tsunami

    early-warning systems can prevent some damage because tsunami waves travel at a very slow

    speed. Seismologists immediately send out a warning when evidence of a large undersea

    earthquake appears on seismographs. Tsunami waves travel slower than seismic P and S waves

    in the open ocean, they move about ten times slower than the speed of seismic waves in the

    rocks below. This gives seismologists time to issue tsunami alerts so that people at risk can

    evacuate the coastal area as a preventative measure to reduce related injuries or deaths.

    Scientists radio or telephone the information to the Tsunami Warning Center in Honolulu and

    other stations.(The floor of the sea p.59) Engineers minimize earthquake damage to buildings by

    using flexible, reinforced materials that can withstand shaking in buildings. Since the 1960s,

    scientists and engineers have greatly improved earthquake resistant designs for buildings that are

    compatible with modern architecture and building materials. They use computer models to

    predict the response of the building to ground shaking patterns and compare these patterns to

    actual seismic events, such as in the 1994 Northridge, California, earthquake and the 1995 Kôbe,

    Japan, earthquake. They also analyze computer models of the motions of buildings in the most

    hazardous earthquake zones to predict possible damage and to suggest what reinforcement is

    needed. (Martin Alfred p.62) Structural Design Geologists and engineers use risk assessment

    maps, such as geologic hazard and seismic hazard zoning maps, to understand where faults are

    located and how to build near them safely. Engineers use geologic hazard maps to predict the

    average ground motions in a particular area and apply these predicted motions during

    engineering design phases of major construction projects. Engineers also use risk assessment

    maps to avoid building on major faults or to make sure that proper earthquake bracing is added

    to buildings constructed in zones that are prone to strong tremors. They can also use risk

    assessment maps to aid in the retrofit, or reinforcement, of older structures. (The ocean of truth

    p.23) In urban areas of the world, the seismic risk is greater in non-reinforced buildings made of

    brick, stone, or concrete blocks because they cannot resist the horizontal forces produced by

    large seismic waves. Fortunately, single-family timber-frame homes built under modern

    construction codes resist strong earthquake shaking very well. Such houses have laterally braced

    frames bolted to their foundations to prevent separation. Although they may suffer some damage,

    they are unlikely to collapse because the strength of the strongly jointed timber-frame can easily

    support the light loads of the roof and the upper stories even in the event of strong vertical and

    horizontal ground motions.(On shifting groung p.73) Emergency Preparedness Plans Earthquake

    education and preparedness plans can help significantly reduce death and injury caused by

    earthquakes. People can take several preventative measures within their homes and at the office

    to reduce risk. Supports and bracing for shelves reduce the likelihood of items falling and

    potentially causing harm. Maintaining an earthquake survival kit in the home and at the office is

    also an important part of being prepared. (On shifting ground p.97) In the home, earthquake

    preparedness includes maintaining an earthquake kit and making sure that the house is

    structurally stable. The local chapter of the American Red Cross is a good source of information

    for how to assemble an earthquake kit. During an earthquake, people indoors should protect

    themselves from falling objects and flying glass by taking refuge under a heavy table. After an

    earthquake, people should move outside of buildings, assemble in open spaces, and prepare

    themselves for aftershocks. They should also listen for emergency bulletins on the radio, stay out

    of severely damaged buildings, and avoid coastal areas in the event of a tsunami. (The floor of

    the sea p.46) In many countries, government emergency agencies have developed extensive

    earthquake response plans. In some earthquake hazardous regions, such as California, Japan, and

    Mexico City, modern strong motion seismographs in urban areas are now linked to a central

    office. Within a few minutes of an earthquake, the magnitude can be determined, the epicenter

    mapped, and intensity of shaking information can be distributed via radio to aid in response

    efforts.(The floor of the sea p.18) STUDYING EARTHQUAKES Seismologists measure

    earthquakes to learn more about them and to use them for geological discovery. They measure

    the pattern of an earthquake with a machine called a seismograph. Using multiple seismographs

    around the world, they can accurately locate the epicenter of the earthquake, as well as

    determine its magnitude, or size, and fault slip properties. (Alfred Wegener & encarta 98) I

    Measuring Earthquakes An analog seismograph consists of a base that is anchored into the

    ground so that it moves with the ground during an earthquake, and a spring or wire that suspends

    a weight, which remains stationary during an earthquake. In older models, the base includes a

    rotating roll of paper, and the stationary weight is attached to a stylus, or writing utensil, that

    rests on the roll of paper. During the passage of a seismic wave, the stationary weight and stylus

    record the motion of the jostling base and attached roll of paper. The stylus records the

    information of the shaking seismograph onto the paper as a seismogram. Scientists also use

    digital seismographs, computerized seismic monitoring systems that record seismic events.

    Digital seismographs use re-writeable, or multiple-use, disks to record data. They usually

    incorporate a clock to accurately record seismic arrival times, a printer to print out digital

    seismograms of the information recorded, and a power supply. Some digital seismographs are

    portable; seismologists can transport these devices with them to study aftershocks of a

    catastrophic earthquake when the networks upon which seismic monitoring stations depend have

    been damaged. (Plate Tectonics p.56-58, 64) There are more than 1000 seismograph stations in

    the world. One way that seismologists measure the size of an earthquake is by measuring the

    earthquake’s seismic magnitude, or the amplitude of ground shaking that occurs. Seismologists

    compare the measurements taken at various stations to identify the earthquake’s epicenter and

    determine the magnitude of the earthquake. This information is important in order to determine

    whether the earthquake occurred on land or in the ocean. It also helps people prepare for

    resulting damage or hazards such as tsunamis. When readings from a number of observatories

    around the world are available, the integrated system allows for rapid location of the epicenter.

    At least three stations are required in order to triangulate, or calculate, the epicenter.

    Seismologists find the epicenter by comparing the arrival times of seismic waves at the stations,

    thus determining the distance the waves have traveled. Seismologists then apply travel-time

    charts to determine the epicenter. With the present number of worldwide seismographic stations,

    many now providing digital signals by satellite, distant earthquakes can be located within about

    10 km (6 mi.) of the epicenter and about 10 to 20 km (6 to 12 mi.) in focal depth. Special

    regional networks of seismographs can locate the local epicenters within a few kilometers. (the

    Ocean of truth) . All magnitude scales give relative numbers that have no physical units. The

    first widely used seismic magnitude scale was developed by the American seismologist Charles

    Richter in 1935. The Richter scale measures the amplitude, or height, of seismic surface waves.

    The scale is logarithmic, so that each successive unit of magnitude measure represents a tenfold

    increase in amplitude of the seismogram patterns. This is because ground displacement of

    earthquake waves can range from less than a millimeter to many meters. Richter adjusted for this

    huge range in measurements by taking the logarithm of the recorded wave heights. So, a

    magnitude 5 Richter measurement is ten times greater than a magnitude 4; while it is 10 x 10, or

    100 times greater than a magnitude 3 measurement. (The floor of the sea p.89-91) Today,

    seismologists prefer to use a different kind of magnitude scale, called the moment magnitude

    scale, to measure earthquakes. Seismologists calculate moment magnitude by measuring the

    seismic moment of an earthquake, or the earthquake’s strength based on a calculation of the area

    and the amount of displacement in the slip. The moment magnitude is obtained by multiplying

    these two measurements. It is more reliable for earthquakes that measure above magnitude 7 on

    other scales that refer only to part of the seismic waves, whereas the moment magnitude scale

    measures the total size. The moment magnitude of the 1906 San Francisco earthquake was 7.6;

    the Alaskan earthquake of 1964, about 9.0; and the 1995 Kôbe, Japan, earthquake was a 7.0

    moment magnitude; in comparison, the Richter magnitudes were 8.3, 8.6, and 6.8, respectively

    for these tremors. (U.S.G.S.) Earthquake size can be measured by seismic intensity as well, a

    measure of the effects of an earthquake. Before the advent of seismographs, people could only

    judge the size of an earthquake by its effects on humans or on geological or human-made

    structures. Such observations are the basis of earthquake intensity scales first set up in 1873 by

    Italian seismologist M. S. Rossi and Swiss scientist F. A. Forel. These scales were later

    superseded by the Mercalli scale, created in 1902 by Italian seismologist Guiseppe Mercalli. In

    1931 American seismologists H. O. Wood and Frank Neumann adapted the standards set up by

    Guiseppe Mercalli to California conditions and created the Modified Mercalli scale. Many

    seismologists around the world still use the Modified Mercalli scale to measure the size of an

    earthquake based on its effects. The Modified Mercalli scale rates the ground shaking by a

    general description of human reactions to the shaking and of structural damage that occur during

    a tremor. This information is gathered from local reports, damage to specific structures,

    landslides, and peoples’ descriptions of the damage. (The road to Jaramillo p.122) II Predicting

    Earthquakes Seismologists try to predict how likely it is that an earthquake will occur, with a

    specified time, place, and size. Earthquake prediction also includes calculating how a strong

    ground motion will affect a certain area if an earthquake does occur. Scientists can use the

    growing catalogue of recorded earthquakes to estimate when and where strong seismic motions

    may occur. They map past earthquakes to help determine expected rates of repetition.

    Seismologists can also measure movement along major faults using global positioning satellites

    (GPS) to track the relative movement of the rocky crust of a few centimeters each year along

    faults. This information may help predict earthquakes. Even with precise instrumental

    measurement of past earthquakes, however, conclusions about future tremors always involve

    uncertainty. This means that any useful earthquake prediction must estimate the likelihood of the

    earthquake occurring in a particular area in a specific time interval compared with its occurrence

    as a chance event. (The ocean of truth p.29) The elastic rebound theory gives a generalized way

    of predicting earthquakes because it states that a large earthquake cannot occur until the strain

    along a fault exceeds the strength holding the rock masses together. Seismologists can calculate

    an estimated time when the strain along the fault would be great enough to cause an earthquake.

    As an example, after the 1906 San Francisco earthquake, the measurements showed that in the

    50 years prior to 1906, the San Andreas fault accumulated about 3.2 meters (10 feet) of

    displacement, or movement, at points across the fault. The maximum 1906 fault slip was 6.5

    meters (21 feet), so it was suggested that 50 years x 6.5 meters/3.2 meters, about 100 years,

    would elapse before enough energy would again accumulate to produce a comparable

    earthquake. (Plate Tectonics) Scientists have measured other changes along active faults to try

    and predict future activity. These measurements have included changes in the ability of rocks to

    conduct electricity, changes in ground water levels, and changes in variations in the speed at

    which seismic waves pass through the region of interest. None of these methods, however, has

    been successful in predicting earthquakes to date. (U.S.G.S) Seismologists have also developed

    field methods to date the years in which past earthquakes occurred. In addition to information

    from recorded earthquakes, scientists look into geologic history for information about

    earthquakes that occurred before people had instruments to measure them. This research field is

    called paleoseismology. Seismologists can determine when ancient earthquakes occurred. (The

    floor of the sea p.118) Seismology, basically, the science of earthquakes, involving observations

    of natural ground vibrations and artificially generated seismic signals, with many theoretical and

    practical ramifications. A branch of geophysics, seismology has made vital contributions to

    understanding the structure of the earth’s interior. (Webster’s) SEISMIC PHENOMENA

    Different kinds of seismic waves are produced by the deformation of rock materials. A sudden

    slip along a fault, for example, produces both longitudinal push-pull (P) and transverse shear (S)

    waves. Compressional trains of P waves, set up by an quick push or pull in the direction of wave

    propagation, cause surface formations to shake back and forth. Sudden shear displacements

    move through materials with slower S-wave velocity as vertical planes shake up and down.

    When P and S waves encounter a boundary such as Mohorovièiæ discontinuity (Moho), which

    lies between the crust and the mantle, they are partly reflected, refracted, and transmitted,

    breaking up into several other types of waves as they pass through the earth. Travel times depend

    on compressional and S-wave velocity changes as they pass through materials with different

    elastic properties. Crustal granitic rocks typically show P-wave velocities of 6 km/sec, where as

    underlying mafic and ultramafic rocks show velocities of 7 and 8 km/sec. In addition to P and S

    waves—body-wave types—two surface seismic waves are the Love waves, named for the British

    geophysicist Augustus E. H. Love, and Rayleigh waves, named after the British physicist John

    Rayleigh. These waves travel fast and are guided in their propagation by the earth’s surface.

    (Plate Tectonics p.142) INTRAMENTS OF STUDY Longitudinal, transverse, and surface

    seismic waves cause vibrations at points where they reach the earth’s surface. Seismic

    instruments have been designed to detect these movements through electromagnetic or optical

    methods. The main instruments, called seismographs, were perfected following the development

    by the German scientist Emil Wiechert of a horizontal seismograph about the turn of the century.

    (Naked Earth p.36-42) Some instruments, such as the electromagnetic pendulum seismometer,

    employ electromagnetic recording; that is, induced tension passes through an electric amplifier

    to a galvanometer. A photographic recorder scans a rapidly moving film, making sensitive

    time-movement registrations. Refraction and reflection waves are usually recorded on magnetic

    tapes, which are readily adapted to computer analysis. Strain seismographs, employing electronic

    measurement of the change in distance between two concrete pylons about 30 m (100 ft.) apart,

    can detect compressional and extensional responses in the ground during seismic vibrations. The

    Benioff linear strain seismograph detects strains related to tectonic processes, those associated

    with propagating seismic waves, and tidal yielding of the solid earth. Still more recent inventions

    used in seismology include rotation seismographs; tiltmeters; wide frequency band, long-period

    seismographs; and ocean bottom seismographs. (Alferd Wegener p.118-120) Similar

    seismographs are deployed at stations around the world to record signals from earthquakes and

    underground nuclear explosions. The World Wide Standard Seismograph Network (WWSSN)

    incorporates some 125 stations. (U.S.G.S.) Richter Who? Richter, Charles (1900-1985),

    American seismologist who wrote fundamental seismology texts, and who established an

    earthquake magnitude scale with German-American seismologist BenoGutenberg. (Encarta 98)

    Richter was born in Ohio but moved to Los Angeles as a child. He attended Stanford University

    and received his undergraduate degree in 1920. In 1928 he began work on his Ph.D. in

    theoretical physics from the California Institute of Technology (Caltech), but before he finished

    it, he was offered a position at the Carnegie Institute of Washington. At this point, he became

    fascinated with seismology. After he worked at the new Seismological Laboratory in Pasadena,

    under the direction of Beno Gutenberg. In 1932 Richter and Gutenberg developed a standard

    scale to measure the relative sizes of earthquake sources, called the Richter scale. In 1937 he

    returned to Caltech, where he spent the rest of his career, eventually becoming professor of

    seismology in 1952. Richter and Gutenberg also worked to locate and catalog major earthquakes

    and used them to study the deep interior of the earth. Together they wrote a very influential

    textbook, published in 1954, called Seismicity of the Earth. In 1958 Richter published the

    textbook, Elementary Seismology, which many consider his greatest contribution to the field.

    Richter visited Japan on a Fulbright Fellowship in 1959-1960. (Encarta 98) Richter was also

    involved in public awareness and safety issues surrounding earthquakes, taking a sensible stance

    rather than using scare tactics. He was devoted to his work in science and learned several

    languages in order to read the global earthquake literature. Richter was so interested in

    earthquakes, he even installed a seismograph in his living room of his Los Angeles home. He

    influenced Los Angeles building codes that city officials credited with saving many lives in the

    1971 earthquake in San Fernando, California. After retirement he continued to work on

    earthquake safety design. (Encarta 98) (PUT MONTH) EARTHQUAKE FINDINGS During the

    month of march we charted all of the bigger earthquakes that occurred . We charted the

    earthquakes measuring from 4 to 7 on the Richter scale. We plotted this data to see where most

    of the earthquakes would occur. Also to see how high most of the quakes would be on the scale.

    According to our analyses most of the earthquakes occurred around the plate boundaries.

    Especially in South America along the South American plate and Mexico along the North

    American plate. Yet, to our surprise there weren’t many earthquakes whatsoever, along the

    boundary between the Eurasian plate and the African plate. We also found Seismic activity in

    some unusual areas like the arctic region above Europe and the Antarctic region. Most of the

    quakes we recorded were not generally large either. Most of them were recorded at 4 on the

    Richter scale. There were not many large earthquakes in the month of March. The largest quake

    we recorded was 6.8 in Xizang-India border region. We also found that there were an unusually

    high number of earthquakes in the month of March. From the data that we collected we noticed

    that earthquakes can also occur in the middle of the ocean. In conclusion from the data we have

    constructed we came to find out that large earthquakes are rare and far in between. We have

    come to realize how devastating earthquakes can really be to people and their surroundings.

    REFERENCES Kidd, J.S. & Kidd, R. A. (1997). On shifting ground “the story of continental

    drift”. New York: Facts on File, Inc. Erickson, J. (1992). Plate Tectonics. New York: Facts on

    File, Inc. Glen, W. (1982). The Road to Jaramillo. Stanford, California: The Stanford University

    Press. Menarld, H.W. (1986). The Ocean of Truth. Princeton, N.J.: The Princeton University

    Press Suhwartzbach, M. (1986). Alfred Wegener. Madison, Wisconsin.: Science Teck Inc.

    Vogel, S. (1992). Naked Earth. New York: Dutton Books. Wertenbacher, W. (1974). The Floor

    of the Sea. Boston Massachusetts.: Little Brown and Co. Internet. (1999).

    wwwneic.cr.usgs.gov/neis/bulletin.html. Computer source.: Internet explorer. Apsell, P. S.

    (Producer). (1990). Nova Earthquake. [Video Tape]. Western Video

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